In some chemical processes, it is necessary to monitor the progress of the chemical reaction and to adjust the supply of the reactants to ensure that the reaction proceeds as desired. The production of acetic acid, which is an important commercial commodity, is one such chemical process. One current method of manufacturing acetic acid, by carbonylation of methanol or its derivatives, such as methyl acetate or methyl iodide, involves a chemical reaction initiated by a Group 9 catalyst system, specifically as an iridium or rhodium coordination compound in the presence of an iodide and water. Carbonylation has become a preferred route to make acetic acid. Nevertheless, there are countervailing considerations which affect implementation of this process. First, the underlying reaction chemistry is intricate, involving a number of interrelated reactions, by-products and equilibria, all of which must be properly balanced, one against the other, to make the process practicable and maximize efficiency of raw material utilization. Also, the catalyst systems, such as coordination compounds of rhodium, iridium and the like, required for carbonylation are generally complex and expensive. Moreover, carbonylation catalyst systems are extraordinarily sensitive to changes in any number of reaction parameters which, in turn, adversely affect catalyst stability and activity.
It is known to manually sample the reactor effluent and perform a separate laboratory analysis of component concentrations using multiple instrumental and wet chemical methods. This procedure is labor intensive and time consuming, resulting in long time lapses between sampling and the characterization of the sample. This method of sample characterization realistically permits generation of a limited number of data points per day, typically about 3 to about 8. Also, because of the delay between sampling and generation of data, the sample characterization would provide an evaluation of a reactor system which may lag behind the actual status of the system by several hours.
Infrared analysis has been used for characterizing components of a chemical process stream. Infrared spectroscopy permits both qualitative and quantitative analyses. Sample analyses can be performed on both organic and inorganic species. Because nearly every molecule has an infrared spectrum, infrared spectroscopy is generally capable of characterizing every molecular component of a chemical process stream without destroying or otherwise modifying the components.
In monitoring the manufacture of acetic acid, the infrared energy absorption corresponding to the stretching frequencies of the hydroxyl and carbonyl groups of acetic acid generates broad absorption bands which tend to overlap, and therefore mask, the infrared bands indicating the presence of a rhodium or iridium catalyst.
In an effort to characterize, for example, rhodium in a rhodium-catalyzed carbonylation system, other methods of analysis have been employed, such as atomic absorption and inductively coupled plasma analysis. However, it is difficult to obtain rhodium concentration data of acceptable precision by either atomic absorption or inductively coupled plasma analysis. Both of these methods involve working up the sample to form a liquid matrix. The process of working up the sample also increases the risk of introducing air into the sample and thereby causing rhodium precipitation. Because of the unreliability of such analyses, the addition of rhodium to the reaction system has been based on an empirical relationship based on carbon dioxide production. However, this empirical relationship is subject to error when other operating conditions are changed, particularly at high operating rates.
It is highly desirable to be able to produce acetic acid under reduced water process conditions without sacrificing catalyst productivity and stability.
Normally, the carbonylation process proceeds at a water level of about 11-14% by weight to maintain the catalyst in its active form. However, that quantity of water must later be separated from the acetic acid produced in the process, increasing processing time and cost. In U.S. Pat. No. 5,817,869 incorporated herein by reference in its entirety, the carbonylation system was modified to achieve low water carbonylation by adding a pentavalent Group 15, formerly Group VA, oxide. Group 15 includes the elements N, P, As, Sb and Bi. Although this new system successfully achieves high yields and reaction rates while stabilizing the active rhodium catalyst component, this modification to achieve low water processing increases the need for a reliable technique to determine the soluble rhodium content.
It is thus desirable to provide a reactant monitoring system that allows for more frequent monitoring of the chemical reaction in the production of acetic acid, particularly where low water processing techniques are utilized. In addition, because of the complexity of the catalyzed carbonylation reaction, monitoring of the catalyst concentration to the exclusion of other reactants is less likely to provide an accurate assessment of the status of the reaction system. It is thus also desirable to be able to monitor and adjust the concentration of up to all of the reactants of the system including the catalyst species based on direct analysis of the reactants. Further, it is desirable to utilize a reactant monitoring system to improve the efficiency of manufacturing acetic acid.